Private investigators involved in a probe into the breach discovered hacking tools, techniques and procedures that were used in earlier cyberattacks that have been linked to Chinese hackers, according to the publication which cited anonymous sources.

These clues suggest Chinese hackers could have orchestrated a campaign with the intention of harvesting data for Beijing's espionage efforts, said the unnamed sources. They also noted that someone else could also have launched the attack because some of the same hacking tools used in the attack were previously made accessible online.

Investigators believe there may have been more than one hacking group inside the guest reservation network for Marriot's Starwood division at the same time. This could make it harder to identify the culprit, as one of the sources noted. It's been four years since the hack was launched in 2014.